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Advantages of PPG sensor | Benefits of Photoplethysmography sensor

This page covers advantages and disadvantages of PPG (Photoplethysmography) sensor. It mentions benefits or advantages of PPG sensor and drawbacks or disadvantages of PPG sensor.

What is PPG (Photoplethysmography) sensor?

PPG is the short form of photoplethysmogram. It converts optical signal to electrical signal to derive various health related parameters. As we know, most of the changes in blood flow occur in arteries and not in the veins. PPG sensor detects changes in blood flow volume using reflected optical signal from the tissue or its transmission through it. It works on the principle that the changes in light intensity is proportional to small variation in blood perfusion.

There are two modes of PPG as follows.
Transmissive mode : In this mode, the light is transmitted through the medium using LED and it is detected by photodiode attached opposite to the LED. The measurement time is limited. It is placed at fingertips, toes or earlobe.
Reflective mode : In this mode, photodiode detects light which is back-scattered or reflected from the tissues, bone and blood vessels. It eliminated problems encountered in mounting of the PPG sensor. It can be placed at forehead or cheek and variety of measurement sites can be used.

PPG sensor Transmissive mode vs Reflective mode

The PPG waveform is represented by two components of current viz. direct current and alternating current. DC detects signals from tissues where as AC detects signals from changes in blood volume.

Benefits or advantages of PPG sensor

Following are the benefits or advantages of PPG sensor:
➨It is simple, reliable and inexpensive.
➨It can easily be integrated in wearable healthcare devices for various health related measurements such as pulse rate (or heart rate), blood flow, Heart Rate Variability (HRV) etc.
➨The use of PPG based wearable devices do not require special training or guidance.

Drawbacks or disadvantages of PPG sensor

Following are the drawbacks or disadvantages of PPG sensor:
➨It can not measure blood pressure.
➨It relies on ECG signal as reference for HR measurement.
➨PPG sensor requires long settling time than ECG sensor.
➨It consumer more power (~30mW) than ECG sensor (~2.5 mW) during operation.
➨It requires external crystal oscillator for accurate timing control.
➨It occupies large PCB footprint.
➨Peak interval accuracy is limited by usable sampling rate due to high power consumption of LEDs.
➨HRV (Heart Rate Variability) requires longer periods of measurement (usually > 5 minutes).
➨It is only suitable for average HR measurement. ECG sensor measures heart rate accurately on beat by beat basis.

Refer ECG sensor vs PPG sensor >> and ECG sensor >> which describes comparison between ECG sensor and PPG sensor with respect to working principles, merits and demerits.

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